1. Field of the Invention
The invention relates generally to the field of electromagnetic induction well logging. More specifically, the invention relates to methods for induction logging using electromagnetic induction well logging instruments having an electrically conductive instrument housing or sonde support.
2. Background Art
Electromagnetic induction well logging is known in the art for determining electrical properties of earth formations penetrated by a wellbore, such as resistivity, dipole constant, and various nuclear magnetic resonance properties, for example. In electromagnetic induction logging, an instrument is lowered into the wellbore. The instrument includes an induction antenna (“transmitter antenna”) coupled to a source of alternating current (AC) having a preselected waveform or a dynamically controllable waveform. Characteristics of the AC waveform, for example, frequency content and amplitude envelope, are selected with respect to the particular properties of the Earth's formations that are being measured. The instrument also includes one or more induction antennas (“receiver antenna(s)”) disposed at axially spaced apart positions along the instrument from the transmitter antenna. Some types of electromagnetic measuring instruments, particularly nuclear magnetic resonance instruments, may use the same antenna for both transmitter and receiver functions. The receiver antenna(s), irrespective of whether they are the same or different than the transmitter antenna, are coupled to circuits which analyze and/or record properties of voltages induced in the receiver antenna(s). Properties of the voltages are analyzed to determine the selected electrical characteristics of the Earth's formations surrounding the instrument. The analyzed properties of the voltages include, for example, amplitude, frequency content and phase with respect to the AC coupled to the transmitter antenna.
A common type of induction antenna, which is used for both transmitter and receiver functions on a typical induction well logging instrument is a so-called magnetic dipole. Magnetic dipole antennas are typically formed as a wire loop or coil. The magnetic dipole moment of the loop or coil is oriented substantially perpendicular to the plane of the loop, or in the case of a coil, substantially parallel to the effective axis of the coil. The loops or coils are typically disposed in appropriate locations on or near the exterior surface of the instrument housing. As a result of the structure of the typical magnetic dipole antenna, the material from which the instrument housing is made becomes important in determining the response of the instrument to the electrical properties of the Earth's formations surrounding the wellbore.
Some types of electromagnetic induction well logging instruments are adapted to be lowered into the wellbore and removed therefrom by means of an armored electrical cable coupled to the instrument housing. This type of instrument is known as a “wireline” instrument. Typically, in wireline induction logging instruments the portion of the instrument housing that includes the transmitter and receiver antennas is made from electrically non-conductive, and non-magnetically permeable material to avoid impairing the response of the well logging instrument to the earth formations surrounding the wellbore.
It is also known in the art to convey well logging instruments into the wellbore as part of a drilling tool assembly (“drill string”). Such “measurement while drilling” (MWD) logging instruments include various forms of electromagnetic induction logging instruments. As a practical matter, MWD logging instruments have steel or other high strength, metallic housings so that the instrument housing can also properly perform the function of a part of the drill string. As a result, the housings of typical MWD well logging instruments are nearly always electrically conductive. See, for example, U.S. Pat. No. 5,757,186 issued to Taicher et al. and U.S. Pat. No. 5,144,245 issued to Wisler. The circuits used in such MWD instruments, and the type of electrical properties measured using such instruments are determined, to a substantial degree, by the presence of the conductive housing (“drill collar”) in such MWD instruments.
It is also known in the art to include high strength, yet electrically conductive support rods inside wireline electromagnetic induction well logging instrument in order to enable such instruments to support the weight of additional well logging instruments coupled below the induction logging instrument. See, for example, U.S. Pat. No. 4,651,101 issued to Barber et al.
It is well known in the art to include a magnetically permeable material, such as ferrite, inside the coil or loop of wire forming a magnetic dipole induction antenna for the purpose of increasing the dipole moment of Such antennas with respect to the selected loop or coil size and configuration. See the previously cited Taicher et al. '186 patent, for example.
It is also known in the art to measure transient electromagnetic characteristics of Earth's formations surrounding a wellbore using a particular type of electromagnetic induction logging instrument. For example, U.S. Pat. No. 5,955,884 issued to Payton et al. discloses an instrument having at transmitter antenna coupled to a source of AC, and electromagnetic and dipole electric receivers antennas disposed on the instrument at locations spaced apart from the transmitter antenna. The AC source has a waveform adapted to induce transient electromagnetic induction effects in the earth formations surrounding the wellbore. The induction receiver and dipole electric receiver antennas detect voltages that are related to transient electromagnetic properties of the formations. It has been impracticable to provide instruments such as disclosed in the Payton et al. '884 patent with a larger electrically conductive housing because conductive housings can reduce the antenna sensitivity to the point where it is difficult to detect sufficient induction signal. Therefore, it has proven impractical for such instruments to be part of the drill string, such as in an MWD well logging instrument.
Geophysical data processing methods known in the art as “deconvolution” have been used to improve the quality of geophysical measurements, including seismic surveying and wellbore logging. Generally speaking, deconvolution can be used to compensate the actual measurements made by a geophysical measurement system for physical limitations of the measurement system. For example, in seismic surveying, it is a goal of seismic data processing to determine, as closely as possible, the seismic reflection response of the Earth to an acoustic “impulse.” An impulse is a theoretical burst of energy having zero time duration, and as a result, essentially equal energy amplitude at all frequencies. Real seismic sources cannot develop such an impulse, if for no other reason than physically embodied apparatus cannot move instantaneously. Deconvolution is used in seismic surveying to adjust the measured reflection response of the Earth with respect to the actual “signature” (waveform) of the seismic energy imparted to the earth. In one example, deconvolution includes generating a “deconvolution operator” which when applied to the actual energy source signature results in as close as possible to an impulse. The same deconvolution operator can then be applied to the measured seismic data to adjust the response. See, for example, E. A. Robinson and S. Treitel, Geophysical Signal Analysis, Prentice Hall (1980) p. 466.
Methods of deconvolution are also known in the art for use in electromagnetic (EM) data analysis, in particular electromagnetic transient induction techniques. See, for example, K. M. Strack, Exploration With Deep Transient Electromagnetics, Elsevier, Amsterdam (1992). Still other deconvolution methods are described in, Ioup, G. E. and Ioup, J. W., Iterative Deconvolution, Geophysics 48, p. 1287-1290, Society of Exploration Geophysicists (1983).
Various forms of deconvolution have also been applied to conventional electromagnetic induction well logging data for some time. See, for example, U.S. Pat. Nos. 4,467,425 and 4,604,581. Deconvolution processing has also been used for electromagnetic propagation well logging, such as used in measurement-while-drilling (MWD) systems. One example of deconvolution for MWD resistivity measurements is disclosed in U.S. Pat. No. 5,329,235. Deconvolution has also been used to improve the results of various nuclear logging processes. See, for example, U.S. Pat. Nos. 5,619,411, 5,672,867 and 5,282,133 (which deals with nuclear magnetic resonance logging).
All of the above methods for deconvolution are generally related to improving the response of measuring systems due to the physical limitations of the energy used to activate the earth in the process of measuring, or to improve the spatial response of the measuring system to adjust for the physical limitations of the measuring system. More particularly, deconvolution as applied to electromagnetic induction well logging as known in the art is generally for the purpose of improving the axial resolution of the induction measuring system.
As explained above, many electromagnetic induction well logging instruments are adapted to be lowered into the wellbore and removed therefrom by means of an armored electrical cable coupled to the instrument housing. Typically, the portion of the instrument housing that includes the transmitter and receiver antennas is made from electrically non-conductive, and non-magnetic material to avoid impairing the response of the well logging instrument to the earth formations surrounding the wellbore. MWD logging instruments also include various forms of electromagnetic induction logging instruments. As a practical matter, MWD logging instruments have steel or other high strength, metallic housings so that the instrument housing can also properly perform the function of a part of the drill string. As a result, the housings of typical MWD well logging instruments are nearly always electrically conductive. Still further, it is also known in the art to include high strength, electrically conductive support rods inside wireline electromagnetic induction well logging instruments in order to enable such instruments to support the weight of additional well logging instruments coupled below the induction logging instrument. See, for example, U.S. Pat. No. 4,651,101 issued to Barber et al.
Because of the desirability of including a conductive support within an electromagnetic induction logging apparatus as explained above, it is desirable to have a method to adjust the response of such instruments for the effect of the conductive sonde support.